Ups and Downs with Marshall Mattress

Within two years, this mattress had a distinct 'mountain range' running down the middle of the bed. When you lie sleep, there is a distinct impression that the bed is trying to roll you off of the edge. Hard work if you try to go uphill into the middle of the bed. 'Technician' came out yesterday to inspect it for warranty coverage. Did some weird measurements with a brightly colored string, as this was the 'industry standard' for determining mattress problems! It was literally like a Monty Python skit. Didn't use his 4 foot level at all. When I examine the mattress using my 7 foot level, it is obvious that the edges are 2 inches lower than the center of the bed. I doubt that the technicians 'magic string' showed this. Will be very surprised if Marshall cover this under warranty. Am looking for another mattress, but will definitely NOT buy a Marshall.